1. Field of the Invention
The present invention relates to a semiconductor device capable of reducing a current leak in an isolation region and a method of manufacturing the same.
2. Description of the Background Art
In a semiconductor device, basic elements such as transistors, capacitors and resistors are connected with wires. Further, electrodes of the transistors are sometimes used themselves as wires. As the electrodes of the transistors and capacitors and the wires, in general, metals such as aluminum and copper and polycrystalline silicon are largely used. In this case, when a polycrystalline silicon film is used as an electrode or a wire, forming a silicide film or a metal film entirely on the polycrystalline silicon film allows reduction in electrical resistance.
A semiconductor film, such as a polycrystalline silicon film, can be used as a resistive element. In this case, the resistance value of the resistive element is in inverse proportion to the cross-sectional area of the polycrystalline silicon film and in proportion to the length and the impurity (dopant) concentration thereof. For example, in order to increase the resistance, the cross-sectional area of the polycrystalline silicon film is made smaller or the length thereof is made longer. Further, reducing the impurity concentration of the polycrystalline silicon film or using the polycrystalline silicon film with no impurity implanted therein increases the resistance.
A polycrystalline silicon film of low resistance to be used as an electrode or a wire and a polycrystalline silicon film of high resistance to be used as a resistive element can be formed of one polycrystalline silicon film. Hereafter, discussion will be made on a method of manufacturing a semiconductor device 1P in the background art, referring to FIG. 14.
First, an isolation insulating film 3P made of a silicon oxide film is formed by LOCOS (Local Oxidation of Silicon) and the like, to section a silicon substrate 2P into an active region and an isolation region. After that, wells and isolation implantation regions are made by ion-implantation and the like.
Next, a gate oxide film is formed in a transistor formation region (now shown). After that, a not-doped (intrinsic) polycrystalline silicon film is deposited by LPCVD (Low Pressure Chemical Vapor Deposition) entirely on the silicon substrate 2P to have a thickness of 50 nm to 250 nm and patterned by photolithography. In this case, a portion of the polycrystalline silicon film which is patterned on the isolation insulating film 3P serves as a resistive element 5P. The resistive element 5P is covered with a resist, an oxide film or the like for protection from any effect in a transistor manufacturing process.
A portion of the patterned polycrystalline silicon film in a transistor arrangement region serves as a gate electrode of a transistor together with a silicide film. Specifically, a metal film such as titanium, cobalt, nickel or tungsten is so formed as to come into contact with the polycrystalline silicon film on an exposed surface thereof and silicified to form a silicide film. Alternatively, a tungsten silicide film or the like is so directly deposited as to come into contact with the polycrystalline silicon film. Further, in this case, the metal film and the silicide film used for forming a gate silicide film are also formed on the resist or the like which covers the resistive element 5P.
After that, forming a protection film, metal wires and the like completes the semiconductor device 1P.
Further, there may be a case where an amorphous silicon is used instead of the polycrystalline silicon.
As discussed above, while the transistor arrangement region undergoes the above processings, the resistive element 5P is covered with the resist or the oxide film. Further, in the processings performed on the transistor arrangement region, the metal film and the silicide film used for forming the gate silicide film are formed on the resist or the like.
The diffusion coefficient of metal atoms in the resist or the oxide film has the same tendency as that in silicon (see FIG. 15), being larger than those of boron, arsenic and the like. For this reason, the metal atoms in the metal film and the silicide film used for the gate silicide film sometimes enter the resist or the like.
The metal atoms entering the resist or the like go through the polycrystalline silicon film 5P or/and the isolation insulating film 3P, to be diffused in the silicon substrate 2P. Consequently, as shown in FIG. 16, metal atoms 11P entering the silicon substrate 2P cause a leak current 12P below the isolation insulating film 3P, in other words, in the isolation region.
Further, metal atoms in the metal film serving as a metal gate (e.g., tungsten or aluminum) and the metal wire (e.g., aluminum or copper) can also serve as the metal atoms 11P.